Paraquat is a herbicide which damages the lungs, liver and kidneys, and produces toxicity by redox cycling with cellular diaphorases, thereby elevating intracellular levels of superoxide (O.sub.2.sup.-). Nitric oxide synthase (NOS) has been shown to participate in paraquat-induced lung injury. Current theory holds that nitric oxide (NO) generated by NOS reacts with O.sub.2.sup.- generated by paraquat to produce the toxic peroxynitrite.